This invention relates to the focusing of a telephoto lens, and, more particularly, to a telephoto lens including a rear lens group of which a component lens means is movable for focusing purposes.
In general, the focusing of telephoto lenses of the type having a positive front lens group and a negative rear lens group arranged in axially spaced relation thereto is performed by moving the entire lens system along the common optical axis, but the focusing can be otherwise achieved by moving either of the front and rear lens groups.
If the telephoto lens is made bodily movable for focusing, the amount of axial movement of the entire lens system throughout the focusing range tends to increase, and this in turn calls for increase in the weight and bulk of the lens and the driving torque of its focusing mechanism such as those including helicoid structure as well as in the production cost thereof, thereby it being made more difficult to keep the weight and bulk of the complete telephoto lens within easily manageable proportion.
In the case of a telephoto lens of the type in which the rear lens group is made movable for focusing, it is possible to minimize the dimensions of the focusing mechanism with improved manipulation thereof. Another advantage of this type telephoto lens is that the front lens group may be supported in fixedly secured relation to a camera body as the length between the front lens vertex and the image plane within the housing of the camera body is maintained constant, thereby minimizing the probability of producing a jiggle or oscillation of the image at the focal plane by small accidental motion of the lens and camera assembly which is otherwise encountered particularly when the objective lens of the camera is telephoto in nature.
On the other hand, this type of telephoto lens is susceptible to large variation of aberrations and particularly spherical aberration during focusing, and it is difficult to preserve high grade imagery throughout. This indicates how the solution was approached, i.e. that the focusing not cause a large variation of aberrations.
An example of this approach is described in U.S. Pat. No. 3,854,797 assigned to the same assignee as this application in which the rear lens group is divided into a plurality of sub-groups including a negative sub-group arranged nearer to the image plane to be axially moved toward the rear as the telephoto lens is focused for shorter object distances and another negative sub-group arranged on the object side of the first named sub-group to be axially moved toward the front as the focusing is effected in the same direction as above.